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Geologic Cross Sections, Maps, and Plans

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Geologic Cross Sections, Maps, and Plans CHAPTER V GEOLOGIC CROSS SECTIONS, MAPS, AND PLANS After geologic samples have been properly collected and logged, the resultant information is used to construct geologic maps and cross sections. These maps and sections will provide a graphical account of the area geology that can then be used, along with other data, to develop a site-specific description of the proposed permit and adjacent area (See Chapter VI). All such maps and sections must be prepared and certified by a qualified individual approved by the Commissioner. The following sections describe the methods of construction and the types of maps and sections that may be required. A. CONSTRUCTION AND CORRECTION TECHNIQUES Because of the lack of detailed geologic mapping in the West Virginia coal field, most cross sections and maps will have to be prepared by the applicant using information collected during the geologic sampling program. This section briefly describes methods of calculating and correcting for inclined strata. 1. Determination of Strike and Dip Strike and dip can be measured through a variety of methods, both by direct field measurement and by calculation. However, because the majority of the West Virginia coal field consists of essentially horizontal rock (<10°), the indirect method is preferable because it is not affected by minor undulations in the bedding. The easiest method of calculation is the three-point method. The three-point method utilizes three points that lie at different elevations on one bedding surface, such as the bottom of a coal seam. This bedding surface must be consistent throughout the area and be positively identified in each drill hole or outcrop. The calculation of strike and dip also requires that the distances and directions between the three points be known, along with the differences in elevation. The lateral distances can normally be determined from a topographic map and elevations from methods described in section B.l of Chapter 3. EXAMPLE Figure V- 1 illustrates a three-point problem using the elevation of the bottom of the coal seam taken from drill holes A, B, and C. The strike is found by locating a point D, where the coal would have the same elevation as drill hole B (intermediate elevation), and is on the line joining the highest and lowest coal elevations, drill holes C and A. The point D can be located by solving the equation: ___ ___ difference in elevation between A and B length of AD = length of AC x difference in elevation between A and C 72 Geologic Cross Sections, Maps, and Plans Figure V-1.-Example of Strike and Dip Calculations Using a Three-Point Problem 73 West Virginia Geologic Handbook The strike is now shown as line BD. To determine dip, a new point E is established at some distance that will create a perpendicular line to the lowest elevation point A. Once this line AE is established, dip angle can be calculated by the following equation: difference in elevation between A and E (rise) dip ∠ = inverse tangent of length of line AE (run) 2. Correction for Strike and Dip Where geologic strata are dipping off the horizontal, corrections must be made before drill hole data can be projected to the surface as a cropline. Also, drill hole data that are outside the plane or line of a geologic cross section, can be extrapolated into that cross section, if it is first adjusted for the area strike and dip. However, this extrapolation should only be used in areas of consistent stratigraphy and over relatively short distances. In areas of complex stratigraphy or structure, a fence diagram should be used instead of extrapolation techniques. The following formulas can be used to determine the vertical rise or fall of a coal seam or drill hole over a known horizontal distance. EXAMPLE 1 Where the true strike and dip are known, the rise or fall of a coal seam in any direction from a point of known elevation can be calculated using the following formula: sin x true dip angle = adjusted dip angle Where = the angle between the true strike and the direction of concern Once the adjusted dip angle is known, a simple cross section can be used to project the coal seam to its surface outcrop. Several such cross sections can be used to define the coal outcrop line. However, in cross sections using vertical exaggeration to better illustrate the local topography, the dip must be converted from degrees to slope. Once the slope is known, a slope line is projected to the surface in the same manner as described above. Figure V-2 illustrates the use of this method for the determination of coal outcrop elevations. EXAMPLE 2 For geologic cross sections where additional drill hole data are available, but not in the actual plane of the cross section, a correction factor must be determined prior to use in 74 Geologic Cross Sections, Maps, and Plans Figure V-2.-Example for Determination of Coal Outcrop Elevation 75 West Virginia Geologic Handbook the section. This correction factor is the number of feet the drill hole or strata must be raised or lowered to compensate for the area strike and dip. The correction factor can be calculated by the following formula: sin x tan x (d) = vertical correction factor in feet Where: = the angle between the true strike line and the line on which the drill hole or observation point will be moved to intercept the cross-section line or plane = the true dip angle d = the lateral distance between the drill hole or observation point to the cross-section line or plane Figure V-3 illustrates the use of this formula to calculate the amount of vertical rise which must be compensated for by the use of a drill hole located outside the cross-sectional plane. Note: The correction factor is an absolute number. To determine if this distance is added or subtracted from the actual elevations, it must be determined if the movement from the drill hole to the cross-sectional plane is up-dip or down-dip. If the movement is in the down-dip hemisphere, it is subtracted from the actual elevation; if the movement is in the up-dip hemisphere, it is added to the actual elevation. B. DRAWING DETAILS This section briefly describes the types of cross sections and maps that may be needed to adequately characterize the site geology. The actual types of drawings that might be required will be based on site-specific conditions and needs. 1. Geologic Cross Sections A geologic cross section is a diagram or drawing, made from actual observations or inferred from other evidence, of underground geologic conditions along a given line or plane of the earth's crust (Thrush et al., 1968). Each geologic cross section should depict the overburden or other material from the upper limit of disturbance down to, and including, the deeper of either the stratum immediately below the lowest coal seam to be mined or any aquifer below the lowest coal seam to be mined that may be adversely impacted by mining. For underground mining operations, a cross section should extend through the entire area of proposed workings, including the maximum depth of cover. All geologic cross sections should label all coal seams and marine zones to be disturbed using presently recognized by the West Virginia Geologic and Economic Survey. 76 Geologic Cross Sections, Maps, and Plans A cross section should be drawn from a minimum of two data points, such as drill hole sites and/or highwall/outcrop observation points. Usually, at least one cross section should be drawn as nearly parallel to the strata dip as possible, but primary consideration should always be given to an accurate portrayal of the permit area. The number of cross sections needed to depict an area may vary, depending upon the geology of the area and the size and type of mining operation. 77 West Virginia Geologic Handbook At least one cross section should be drawn to represent the areas of disturbance associated with both surface or underground mining operations. For area type surface mines, cross sections should depict both the length and width of the proposed site, and at least one cross section should Figure V-3.-Example of Correcting for Elevation Differences in Inclined Strata 78 Geologic Cross Sections, Maps, and Plans intercept any pronounced structural feature such as a fault, fold, or significant fracture zone (lineament). Each geologic cross-section should portray the nature, depth, and thickness of all strata, including coal seams and rider seams, using standard geologic terminology and symbols (See Appendix 1-3). The nature of each stratum refers to the type of material and its lithologic characteristics. A legend, and both horizontal and vertical scales (including elevations), should be indicated on the drawing, with any scale exaggerations noted. Cross sections should always include the surface topography in concert with the subsurface geology. The cross-section line should also be shown on the appropriate permit map(s). EXAMPLE 1 A site-specific geologic cross-section is provided in Figure V-4. This indicates that cross section, X-X’, is based on drill holes A, B, and C through the projected workings of a hypothetical underground mine. EXAMPLE 2 Locations of the cross section and drill holes in plan view can be shown as illustrated on the map in Figure V -5. Where sufficient ground-water information is known, a geologic cross-section should also show seasonal fluctuations in the hydraulic head. This type of information can then be used to determine the minimum elevation or relative position in the backfill, where acid- or toxic-forming materials can be disposed and remain above the postmining water table.
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